Remove CSS helper classes

The following CSS helper classes get removed from `main.css`:

* Image replacement: `.ir`
* Hidden elements: `.hidden`
* Visually hidden elements: `.visuallyhidden`
* Visually hidden elements that are focussable via keybord:
`.visuallyhidden.focusable`
* Invisible elements: `.invisible`
* Clearfix: `.clearfix`

These classes were removed because they don't provide enough use cases within
modern web development. Cases are mostly handled indiviually by developers.

All the helper code is retained in the [`v4`](https://github.com/h5bp/html5-boilerplate/tree/v4) branch.

Reference #1472.

Please feel free to provide feedback and discuss this commit here.